1. Field of the Invention
The present invention relates to liquid crystal devices used in electrophotographic printers as light valves and, more particularly, to those transmissive, nematic types arranged as a single array of dot shutters which have response times fast enough to form latent electrostatic images on a moving photoconductive member one line at a time.
2. Description of the Prior Art
In a typical transmissive type, liquid crystal device, a thin layer of liquid crystal material is sandwiched between parallel, transparent glass substrates bearing transparent, patterned electrodes on their inner confronting surfaces. At least one polarizer is located on the outer surface of one of the glass substrates and a light source spaced from the device directs light therethrough. By selectively applying an electric field across the layer of liquid crystal material by means of selectively applying an a.c. voltage to the electrodes, the transmissivity of the liquid crystal device may be changed for passing or blocking light in accordance with the electrodes addressed by the voltage.
Liquid crystal materials are organic substances made up of rod-like molecules that are typically about 10 angstroms long and several angstroms thick. Within certain temperature ranges, these materials exhibit optical properties of an ordered crystal but have flow properties of liquid.
There are three general types of liquid crystal materials; viz., the smectic, the nematic, and the cholesteric. These are distinguished by differing types of translational or orientational ordering in their molecular arrangement. In the nematic-type, which is the type used in the present invention, the center of gravity of the molecules is unordered and random, as in the case of isotropic liquids, but the molecules tend to align themselves with their long axes parallel.
Since the individual liquid crystal molecules have a elongated shape and dipoles (both permanent and induced) which are direction dependent, films of these materials exhibit anisotropy in their dielectric constant and refractive index. Materials that exhibit a positive dielectric anisotropy have molecules that tend to align themselves parallel to an applied electric field, while the molecules of materials that exhibit a negative dielectric anisotropy tend to align themselves perpendicular to the field. Because of their optical anisotropy, a change in orientation of the liquid crystal molecules by an electric field can cause a change in optical transmission when used in conjunction with light polarizing sheets.
By suitable treatment of the inner glass substrates, nematic liquid crystal material which have a positive dielectric anisotropy are caused to align in a particular direction parallel to the glass substrate surfaces. In one method, the glass substrates may be coated with a thin organic film and conditioned by, for example, rubbing it with a lint-free cotton twill cloth in a unidirectional manner. Fine grooves about 50 angstroms wide are formed causing the liquid crystal molecules to lie substantially parallel to these furrows since this results in a lower energy state. Such a conditioned film is generally called the alignment layer or film. During fabrication the two glass plates are oriented with their alignment directions (in this invention) parallel to each other.
A typical transmissive, liquid crystal device that uses a nematic liquid crystal material with a positive dielectric anisotropy comprises two parallel, transparent glass substrates having one or more transparent electrodes on the inner surface of a one of the glass substrates with a plurality of transparent electrodes on the inner surface of the second glass substrate that are parallel to each other but perpendicular to the electrodes on the inner surface of the first glass substrate. A transparent alignment layer covers the electrodes so that the molecules of the liquid crystal material placed between the glass substrates are parallel to the glass substrate surface while they are on their stable relaxed state. When a voltage is applied to the electrodes, the molecules orient themselves perpendicular to the glass substrates and parallel to the direction of the electric field. A polarizer placed on the outside surface of one of the glass substrates, allows the light vector of one direction to pass therethrough but blocks all other light vectors.
When the liquid crystal molecules are lying parallel to the glass substrates in their relaxed state, due to their birefringence, they convert the linearly polarized light passed by one polarizer into elliptically polarized light. Once a voltage is applied to the electrodes, the molecules rotate by 90 degrees to align themselves parallel to the electric field and perpendicular to the glass substrates. This is the electrically driven stable state, and in this arrangement, it does not affect the state of polarization of light that travels in a direction essentially perpendicular to the glass substrates. If two polarizers are used on either side of the device, their transmission axes can be either parallel or crossed depending on whether it is desirable that the driven state be clear or dark respectively.
Liquid crystal devices generally change states from transmissive to non-transmissive and back again to its original state relatively slowly. Their response times are typically 100 milliseconds or longer and, at that rate, would require at least five minutes to print one page, if a typical liquid crystal device having a single array of dot shutters at a density of 10 per millimeter were used as an image bar to mark the photoconductive member of an electrophotographic printer.
Liquid crystal devices react slowly because they rely on the motion of long, heavy molecules in a viscous environment. Nearly all present devices currently used are based on the existence of two optically distinct states, one of which is an electrically driven state and the other is a relaxed or at rest state. The transition time from the relaxed state to the driven state can be made short provided that the applied voltage is high enough. However, the return to the relaxed state is a slow transition which is controlled only by the coupling forces that bond the molecules to the inner face of the device electrodes. The problem of the slow return from the driven state to the relaxed state is always present.
Since the electrically driven state may be achieved very fast, one solution to the problem of slow recovery to the relaxed state consists of driving the liquid crystal material from the driven state back to the relaxed state. This is the dual frequency approach requiring a liquid crystal material which orients its molecules in one direction when a first electric field of one frequency is applied and in another direction 90 degrees from the first when a second electric field of a second frequency is applied. Such liquid crystal materials are very temperature dependent and require means to keep their temperatures constant. In addition to this extra apparatus, the electronic circuitry of a dual frequency method can be complex and costly.
U.S. Pat. No. 3,854,751 to W. E. Haas and J. E. Adams discloses a device with interdigitated electrodes, in which two electric fields perpendicular to each other can be applied sequentially to turn the liquid crystal molecules parallel or perpendicular to the glass substrates. With this scheme the long relaxation times can be avoided, but for high resolution applications, as required of image bars for electrophotographic printers, the approach is too complex and costly to manufacture to be considered in all but the most expensive printers.
U.S. Pat. No. 4,386,836 to K. Aoki et al discloses a liquid crystal image bar which operates as a light valve. The liquid crystal material has an inverted dielectric anisotropy on opposite sides of a critical frequency and is selectively driven from one stable state to another by selectively applying two different frequency signals to the liquid crystal material.
U.S. Pat. No. 3,857,629 to M. J. Freiser and U.S. Pat. No. 4,009,934 to R. M. Goodwin et al also shows liquid crystal devices which use the dual frequency method to drive the liquid crystal material between stable states. An article entitled "Fast-Switching Twisted Nematic Electro-Optical Shutter and Colour Filter" by J. P. Sumner, Electronics Letters, Vol. 10, No. 7, 4/4/74, Pages 114 and 115, discloses yet another dual frequency method.
U.S. Pat. No. 3,697,150 to J. J. Wysocki discloses a twisted or cholesteric cell with a dipolar material added to the liquid crystal material to reduce the time during which the liquid crystal material reaches the stable relaxed state from the stable electrically driven state. Since this patent is concerned with moving from one stable state to another, the response time are too slow for use in electrophotographic printer image bars.
U.S. Pat. No. 3,694,053 to F. J. Kahn discloses a nematic liquid crysta device having an alignment layer and an electronically tunable optical birefringence which may switch colors by changing the voltage applied. This patent teaches moving from one stable state to another stable state so that its response times may not be suitable for use as an image bar, furthermore, the tolerances within which the devices must be made are very tight.
U.S. Pat. No. 3,821,720 to W. Greubel et al discloses a complex system for storing displayed information in a liquid crystal device using a cholesteric or twisted cell having a positive dielectric anisotropy. The device operates from one stable state to another and, therefore, has response time too slow for use in image bars.
U.S. Pat. No. 3,785,721 to T. B. Harsch discloses a nematic liquid crystal device which varies color by varing the applied voltage. This device is not concerned with response time and, of course, cannot be used as an image bar.
U.S. Pat. No. 3,784,280 to J. E. Bigelow discloses a reflective, light-dark nematic liquid crystal device in which the liquid crystal material has a positive dielectric anisotropy. Molecule alignment at the opposing transparent substrate surfaces are mutually parallel. High voltage orients the liquid crystal molecules in the electric field to a position perpendicular to the substrate surfaces, thus readily transmitting light and becoming the bright state. By utilizing a polarizer at a 45 degree angle with respect to the liquid crystal molecules in their relaxed state on one side of the liquid crystal device and a quarter wave plate and reflector on the other side, one polarizer may be eliminated to increase intensity of the bright state. This device operates from stable states and does not teach improvement in response time between states. Also, this device is a reflective one and is not applicable as an image bar which generally is a light transmissive device.
U.S. Pat. No. 4,097,128 to S. Matsumoto et al discloses a nematic liquid crystal cell in which the liquid crystal material contacting one of the cell substrate surfaces orients the material molecules parallel to the substrate surface, while the other substrate surface orients the material molecules perpendicular thereto. An applied electric field rotates the molecules of the liquid crystal material to change the birefringence of the liquid crystal material and thus the color of light displayed. Extreme gap dimensional control or tolerance is required to achieve a single color with various voltage levels applied to the electrodes of the device. Also, the device operates from one stable state to the other and rapid response times are not critical as is necessary for image bars.
U.S. Pat. No. 4,126,382 to G. Barzilai et al discloses a method for displaying images with liquid crystal devices. It is observed that from a certain value of the RMS applied voltage pulse, the intensity of the transmitted light is rapidly increased up to a peak value and then decreases with appearance of other luminosity peaks which may correspond to transient states. Since the human eye has a slow response time, it is fooled into seeing white light as the liquid crystal molecules remain in dynamic deformation conditions (transient state) between applied voltage pulses. This patent is directed to displays such as TV screens and does not address the problem of the fast response times necessary for image bars which digitally produce a linear array of light spots on a moving photoconductive member to produce a latent electrostatic image one line at a time. To the contrary, Barzilai wants relatively slow transition time since it is dealing with human perception.
An article entitled "A LC/CRT Field-Sequential Color Display" by R. Vatne et al, Society for Information Display (SID), Digest of Technical Papers, May 1983, Pages 28 and 29, describes a color system that uses a new type of liquid crystal color filter. This system uses a single frequency material in a liquid crystal device called a ".pi.-Cell" for fast switching and excellent angular viewability.
An article entitled "A Liquid Crystal Optical Switching Devices (.pi.-Cell)" by P. J. Bos et al, Society for Information Display (SID), Digest of Technical Papers, May 1983, Pages 30 and 31, describes a new approach to make a fast, liquid-crystal, optical-switching device having a large cone of view.
The two SID articles discuss a thin liquid crystal cell on the order of 3 to 5 .mu.m in order to provide a large viewing angle, and a .pi.-Cell configuration is used to minimize the response time.